1. Field of the Invention
The present invention relates to an information recording system and information recording method for recording information on an optical disc, and particularly to the technique of precisely generating a reference clock signal for use in, for example, precisely determining a recording position when recording information on a wobbled optical disc such as a DVD-R.
2. Description of the Prior Art
When recording data on a DVD-R, information recording systems detect a push-pull signal including a wobble frequency component of an information track by applying a light beam to the information track. Part of the light beam happens to be applied to pre-pits on tracks adjacent to the information track, which causes signals corresponding to the pre-pits to be incorporated with the push-pull signal detected from the information track, resulting in a problem of not being able to detect a quality push-pull signal.
The signals corresponding to the pre-pits have impulse characteristics, i.e. characteristics to have a signal uniformly distributed over the entire frequency band, and thus, they cause another problem of not being able to remove the signals corresponding to the pre-pits only by a simple filter.
Japanese Laid-Open Patent Publication No. 10-241260 discloses an information recording system free from such problems.
The information recording system disclosed in the publication includes a wobble signal extracting unit 22. The wobble signal extracting unit 22 includes, as shown in FIG. 1 in the publication, a limit-level generator 25, a limiter 26, a BPF 27, and a binarizer 28.
The limit-level generator 25 generates upper limit signal SUL and lower limit signal SWL, which may be modulated according to pre-information signal SPP and limit the level of SPP, based on pre-information signal SPP, i.e. a push-pull signal, outputted by a pickup. The width between upper limit signal SUL and lower limit signal SWL is set to a predetermined level width.
The limiter 26 uses the limit signals generated by the limit-level generator 25 to suppress a noise signal derived from pre-pits in pre-information signal SPP and outputs limited pre-information signal SPL.
The BPF 27 removes a high frequency component from the limited pre-information signal SPL to output wobble signal SB.
The binarizer 28 binarizes wobble signal SB to output extracted signal SDTT.